Superposition

Question 1

State the principle of superposition

Answer 1

When two or more waves of the same type meet at a point, the resultant displacement is the algebraic sum of the individual displacements

Question 2

Define

stationary wave

Answer 2

A wave pattern produced when two progressive waves of the same frequency travelling in opposite directions combine. It is characterised by nodes and antinodes. Also known as a standing wave.

Question 3

In a stationary wave, what are nodes and anitnodes?

Answer 3

Nodes:

• a point on the wave with zero amplitude due to destructive superposition, i.e. at this point the waves always have a phase difference of 180^o or π

Antinodes:

• a point vibrating with maximum amplitude due to constructive superposition, i.e. at this point the waves always meet in phase

Distance between 2 successive nodes / antinodes = λ / 2

Question 4

Define

diffraction

Answer 4

The spreading of a wave when it passes through a gap or past the edge of an object.

For maximum diffraction to occur, the size of the gap should be equal to the wavelength of the wave.

Question 5

Define

interference

Answer 5

The formation of points of cancellation and reinforcement where 2 coherent waves pass through each other

Question 6

Define

coherence

Answer 6

Two sources are coherent when they emit waves with a constant phase diff erence.

Note: two waves can only have a constant phase difference if their frequency is the same and remains constant

Question 7

List the conditions required if two-source interference fringes are to be observed

Answer 7

• Must be coherent/meet at a point
• Must be of the same type
• Must be polarised in the same plane or unpolarised
• For a well-defined interference pattern, should have the same amplitude